The present invention generally relates to a composite sandwich structure having outer fiber reinforced composite layers separated by a lightweight core. More particularly, the present invention relates to integrally reinforcing a section of the composite sandwich structure, e.g., by changing the thickness or proportions of one or more layers of the sandwich structure, to provide a stiffened section for a structural interface. The invention has particular advantages for constructing sidewalls of launch vehicles or other aerospace applications.
Reinforced composite sandwich structures typically have outer fiber reinforced composite layers separated by a lightweight core made up of metallic or non-metallic honeycomb, structural foams and/or wooden fibers. The outer fiber reinforced composite layers, or face sheets, are generally separated by and connected to the core, which is usually less stiff and less dense than the face sheets. These composite sandwich structures are widely used today in aerospace applications due to their high stiffness-to-weight (i.e., specific stiffness) and strength-to-weight (i.e., specific strength) ratios. The face sheets generally comprise a fiber reinforced resin matrix composite that incorporates strong stiff fibers, such as carbon fiber, into a softer, more ductile resin matrix. The resin matrix material transmits forces to the fibers and provides ductility and toughness while the fibers carry most of the applied force. In the case of composite sandwich structures, the behavior of the face sheets is analogous to the flange of a structural I-beam while the behavior of the core is analogous to the web of the I-beam. In this regard, the face sheets carry the applied loads and the core transfers the load from one face sheet to the other.
Though composite sandwich structures provide increased strength-to-weight ratios compared to, for example, metallic structures, there are several important limitations to use of such composite structures. Composite structures depend primarily on the fiber reinforcement in the resin matrix for their high specific strength and stiffness. These composite structures generally have limited in-plane compressive strength (bearing strength) and may not have the strength to absorb highly localized stress loads, especially when those loads are applied substantially perpendicular to the composite structure. For example at a structural interface a fastener, such as a bolt, passing through the cross sectional area of a composite sandwich structure may provide a localized stress concentration and/or a point load on one or both of the face sheets. In this regard, the composite sandwich structure must provide adequate bearing strength and compressive strength to resist tearing of the face sheets and/or crushing of the core while providing required structural properties to distribute the point load across the structure""s surface without failing.
In order to provide the necessary structural integrity necessary at, for example, structural interfaces, additional composite material layers are typically added to the face sheets of the composite sandwich structure. These additional layers, or doublers, provide increased stiffness and bearing strength to the structural interface. Generally, to provide the necessary structural integrity, both face sheets are reinforced with doublers. The additional layers increase the weight of the composite structure, thus reducing the specific strength and stiffness benefits provided by the composite sandwich structure. Therefore, only the region surrounding the structural interface is xe2x80x9cdoubledxe2x80x9d, allowing the rest of the composite structure to maintain its high strength-to-weight and stiffness-to-weight ratios.
In the case of large tubular composite structures, as are used for various components of space launch vehicles, doublers may be applied in one or more ways. For example, the doublers may be co-cured on the outside of composite face sheets, which requires the doublers be applied during the initial composite structure xe2x80x9clay-up.xe2x80x9d As will be appreciated, tubular composite structures are generally formed or laid-up on a mandrel that is removed after the structure is cured. In the case of tubular composite sandwich structures, adding doublers during lay-up requires a stepped mandrel having a varied diameter along its length. The stepped mandrel allows a doubling layer to be wound about the mandrel and then the normal face sheet layer wound on top of the doubling layer. As will be appreciated, if the double layer on the inside surface of the tubular structure is in any position other than the end of the mandrel, or if two doubling layers are utilized along the length of the mandrel, the mandrel cannot slide out of the composite sandwich structure upon curing. In this regard, a collapsible mandrel must be used. However, collapsible mandrels increase the cost, weight, and internal structure required of the mandrel, creating difficulties in maintaining mandrel stiffness and tolerances and further creating difficulties in the machinery utilized to apply the materials to the mandrel.
A second method for adding doublers to a section of a tubular composite sandwich structure involves post-bonding the doublers onto a pre-cured structure""s face sheets. This allows a mandrel to be removed from a tubular composite structure prior to application of the doublers. However, adding the doublers, especially to the inside surface of a tubular structure, requires extensive tooling and costs. Further, care must be taken to assure the secondary bonding of the doublers to the face sheets provides good mechanical conformance. As will be appreciated, if the doublers do not properly adhere to the surface of the pre-cured face sheets such that, for example, internal voids exist, the entire composite sandwich structure may be irreparably damaged.
Finally, another method for providing a structural interface for a composite sandwich structure is to pan down the ends of the composite sandwich structure such that it transitions from a sandwich construction having two face sheets and an internal core to a monocoque construction where there is no core and the face sheets are now in direct contact. However, this eliminates many of the benefits of utilizing a composite sandwich structure, e.g., I-beam behavior and increased moment of inertia. Further, monocoque transition requires additional tooling and fabrication steps.
All of the above noted methods for providing doublers to stiffen a section of a tubular composite sandwich structure require substantial tooling and manufacturing steps, increasing the cost of the composite structure. Further, each of the above noted methods changes the external geometry of the composite structure (i.e., the spatial envelope within which the structure is contained defined by its exposed surfaces, which may define interior or exterior walls of an aerospace structure) relative to an un-reinforced geometry, which may be problematic in space launch vehicles.
It is therefore an objective of the present invention to provide an integrally reinforced section in a composite sandwich structure for use as a structural interface.
It is a further objective of the present invention to provide a process to produce an enhanced structural interface in a composite sandwich structure that does not require the use of specialized mandrels or tooling in the lay-up process.
It is a yet further objective of the present invention to provide an enhanced structural interface in a section of composite sandwich structure without altering the exterior dimensions of that composite structure
It is a yet further objective of the present invention to provide a method for allowing selective alteration of the structural properties of a composite sandwich structure for a given exterior geometry limitation.
One or more of the above-noted objectives, as well as additional advantages, are provided by the present invention, which includes a composite sandwich structure having a first face sheet, a second face sheet, and a core sandwiched between the face sheets. More particularly, for a given external geometry, reinforcement of a section of interest is achieved by varying the proportions of the thicknesses of the structural layers. For example, the composite structure may contain at least first and second sections having equal cross-sectional thicknesses measured from the outside surfaces of each face sheet while the relative proportions of the core relative to at least one of the face sheets vary between the first and second sections. As will be appreciated, by varying the relative proportions of the face sheet(s) and core, a composite structure having varying mechanical properties between the first and second sections may be produced while maintaining a predetermined outside profile. Particularly, the relative proportions of the first and/or second face sheet and core may be varied to produce a section within the composite structure that is structurally enhanced (i.e., integrally reinforced) in comparison to other sections of the composite structure. This structurally enhanced section may then be utilized as, for example, a structural interface or joint for attaching the composite structure to other structures.
According to a first aspect of the present invention, a structure for use as a portion of sidewall of space launch vehicle is provided that includes a first face sheet, a second face sheet, and a core sandwiched between the inside surfaces of the first and second face sheets. This core has at least first and second thicknesses at first and second positions along the length of the structure. While the thickness of the core changes between the first and second positions, the distance between the outside surfaces of the first and second face sheets remains substantially equal at the first and second positions. As will be appreciated, in order to maintain the substantially equal distance between the outside surfaces of the face sheets at the first and second positions, the thickness of the first and/or second face sheet generally changes in proportion to the change in the core thickness.
Various refinements exist of the features noted in relation to the subject first aspect of the present invention. Further features may also be incorporated in the subject first aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. For instance, the shape of the composite structure and the locations of the first and second thicknesses of the core may each be varied. In the case of the shape of the structure, a tubular structure may be particularly apt for use as a sidewall in the space launch vehicle. Alternatively, separate curved or flat panels may also be utilized to form the launch vehicle""s sidewall. In the case of the core thicknesses, the first and second core thicknesses may be uniform, for example, across the entire width (i.e., about the circumference in a tubular structure) of the composite structure in first and second positions along the length of the structure. Alternatively, the first and second core thicknesses may only extend across a portion of the structure or be formed around regions where altered structural properties of the composite structure are desired.
The face sheets may be any material that provides the structural, thermal, and other properties desired for the sidewall of the launch vehicle. Preferably, at least one of the face sheets has a first thickness in a first position and a second different thickness in a second location to correspond with the first and second core thicknesses. This change in face sheet thickness allows the distance between the outside surfaces of the first and second face sheets to be equal in the first and second positions along the length of the structure without internal gaps or other accommodations in relation to the varying core thickness. As will be appreciated, an increased thickness section of either or both face sheets will generally stiffen the structure and provide greater bearing strength than thinner sections of the face sheets. Therefore, a preferred embodiment for structural interfaces utilizes a reduced core thickness and a corresponding increase in thickness for one, and more preferably both, face sheet(s), providing a section in the composite structure having enhanced stiffness and bearing strength.
In a preferred embodiment, the face sheets are made from one or more layers of a fiber reinforced material. That is, the face sheets may be made of a material that utilizes strong stiff fibers encapsulated in a softer, more ductile resin matrix. The face sheets may be formed from a plurality of layers of carbon fiber reinforced plastic, glass fiber reinforced plastic, aromatic polyamide fiber (such as Kevlar(copyright) made by DuPont) reinforced plastic, or any other appropriate material. In this regard, the thickness of each face sheet may be varied by varying the number of fiber reinforced material layers used to form that face sheet. For example, additional fiber reinforced material layers may be added to or removed from one or both face sheets in positions where there is a corresponding reduction, or increase, in core thickness. More particularly, these xe2x80x9caugmentationxe2x80x9d layers may be applied to or removed from what becomes the inside surface of the face sheets (i.e., the side in contact with the core). By applying and removing the augmentation layers to the inside surfaces of the face sheet(s), the relative proportions of the face sheets and core may be altered without altering the outside dimension of the composite structure.
The structure""s core may be any material that provides, in conjunction with the face sheets, the structural, thermal and other properties desired for the sidewall of the launch vehicle. A non-inclusive list of appropriate materials include any light-weight material such as metallic (e.g. aluminum) or non-metallic (e.g. Nomex manufactured by Crxc3xa9ations Guillemot Inc. of Beauport, Quxc3xa9bec, Canada) honeycomb, structural foam, balsa wood, a metal or metal alloy in an appropriate form, a metal matrix composite in an appropriate form (e.g., a hybrid of a metal/metal alloy and one or more non-metallic materials), or any other appropriate core material and in any appropriate form, including solid materials. As noted, the core has first and second thicknesses in first and second longitudinal positions along the length of the composite structure. In this regard, the core may contain one or more xe2x80x9cstepsxe2x80x9d on one or both of its surfaces. That is, one side may remain substantially planer between the first and second positions while the other side of the core varies the core""s thickness between the first and second steps. As will be appreciated, in this situation, the face sheet on the varying side of the core may correspondingly vary in thickness between the first and second positions while the face sheet on the planer side of the core may remain a constant thickness between the first and second positions.
The structural properties of the core may also vary along the length of the structure. For example, core properties at the first longitudinal position may be different than core properties at the second longitudinal position. In one preferred embodiment, the density between the two positions varies with structural requirements. For example, to increase bearing and compressive strength at a reduced thickness core position for use in a structural interface, a core having an increased density may be utilized. The change in density from the first and second position may require using a material having varying properties (i.e., a denser portion along its length) or using two separate materials, such as a solid aluminum block (denser) in a reduced core thickness position having high bearing and compressive strength requirements, and an aluminum honeycomb in core positions having greater thickness and lower bearing and compressive strength requirements. Regardless of the core materials utilized, it is preferred that the first and second sections of these materials are somehow xe2x80x9cknittedxe2x80x9d together, (i.e., glued, welded, etc.) to increase the structural properties of the resulting structure.
In the case of a tubular structure used in the sidewall of a space launch vehicle, one or more integrally reinforced sections may be utilized. Particularly, the ends of such a structure may be integrally reinforced to provide a structurally sound joint to attach the structure to other components of the launch vehicle. However, it will be appreciated that one or more integrally reinforced sections may be utilized along the length of sidewall.
According to a second aspect of the present invention, a process for making a structure for use as a portion of a space launch vehicle""s sidewall is provided. Steps of the process include applying a first face sheet to the outside surface of a mandrel; covering the resulting outside surface of the first face sheet with a core layer having at least a first and second thickness at first and second positions along the length of the mandrel; applying a second face sheet to the resulting outside surface of the core layer; curing the resultant structure and removing the mandrel. Further, at least one of the face sheets will be applied having at least first and second thicknesses such that the combined thickness of face sheets and core (i.e., relative proportions) at each the first and second positions is equal.
Various refinements exist of the features noted in relation to the subject second aspect of the present invention. Further features may also be incorporated in the subject second aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. For instance, a tubular mandrel having a uniform outer surface such as a cylinder may be utilized to form the structure; the mandrel may taper from its first end to its second end; the mandrel may contain steps on its surface to provide for variations in the face sheet formed thereon, etc. The methods of application of the face sheets and/or the core may also vary. For example, the face sheets may comprise strands or filaments that are machine placed (e.g., wound or fiber placed) onto the mandrel, material that is applied by hand, or a combination of the two.
The first and second applying steps, in a preferred embodiment of the present invention, include applying a plurality of fiber reinforced material layers to the outside surface of the mandrel and core, respectively. These fiber reinforced material layers may comprise broad goods (sheets) or filaments. Further these layers will contain a resin that forms the face sheet into a solid laminate structure upon curing. This resin may be a wet resin applied to the fiber reinforced materials after or during placement, or, more preferably, the fiber reinforced materials will be pre-impregnated with the resin material prior to application. As will be appreciated, fiber reinforced materials generally contain an xe2x80x9caxisxe2x80x9d along which the fibers are oriented. In each applying step, the various fiber reinforced material layers may be applied such that the axes between the layers are orthogonal to produce desired mechanical, thermal and/or other desired properties.
In each of the first and second applying steps, where a plurality of fiber reinforced material layers are utilized, the number of layers may be varied between first and second positions of the face sheet(s) so as to change the thickness of the face sheet in these first and second positions. That is, at least one of the face sheets may have extra fiber reinforced material layers xe2x80x9cwoundxe2x80x9d onto the mandrel or core (or otherwise applied) in positions corresponding with a reduction in thickness of the core along the length of the mandrel. By inserting extra layers onto what becomes the inside surfaces of the face sheets during lay-up, the overall thickness of the composite structure may remain equal in the first and second positions corresponding with the first and second core thicknesses without changing the outside dimensions of the structure.
The step of curing the composite structure generally entails consolidating the composite structure and causing the resin material to harden/set and or dry. Consolidation may utilize any method to provide a compressive force to the face sheet layers such that there is good conformance between the layers upon curing. A non-inclusive list of consolidating method includes, vacuum bagging, pressurized chambers, compaction rollers and squeegees (e.g., for wet resin applications). Curing is preferably done in a autoclave that heats the structure in a pressure chamber having an elevated pressure. Preferably, the composite structure is both vacuum bagged and cured in a pressurized autoclave to provide increased consolidating force for the multi-layered face sheets. The exact temperature and pressure settings as well as the duration of the curing process varies in relation to the materials utilized to form the composite structure.
According to a third aspect of the present invention, a method for designing a multi-layered structure having at least one reinforced portion is provided. The method includes the step of determining a spatial envelope for the structure. That is, determining at least one constraining factor, such as a maximum allowable thickness or length for the multi-layered structure. Next, a portion of the structure is identified for reinforcement. The portion identified for reinforcement may be so identified for any of a plurality of reasons, typically, reinforcement is desired due to the forces that are expected to act upon the structure during its intended use. Based on the constraints of the spatial envelope, at least one of the material properties and/or the dimensions of one or more layers of the multi-layered structure are altered to provide the desired reinforcement at the identified portion. That is the properties/dimensions of the identified portion""s layers are altered in comparison with, for example, the layers of non-reinforced portions of the structure. Finally, the reinforcement of the desired portion is designed such that it does not create an external irregularity on a surface of the multi-layered structure.
Various refinements exist of the features noted in relation to the subject third aspect of the present invention. These refinements and additional features may exist individually or in any combination. For example, the step of identifying a portion of the structure for reinforcement may comprise determining one or more required structural properties of the portion to be reinforced, such as tensile strength, compressive strength, bearing strength, stiffness, or any other desired property (.e.g., thermal properties, etc). As will be appreciated, the spatial envelope may also be used to determine the required structural properties of the portion to be reinforced. For example, a planer multi-layered structure having maximum allowable thickness of N (i.e., spatial envelope) and a point load of X may require reinforcement such that the load does not deflect the structure beyond a predetermined maximum value.
As will be appreciated, the method of designing the multilayered structure with one or more reinforced portions requires that the material properties of the various layers be known in order to determine the structure""s overall properties. Once the general properties of the structure are known, a material property and/or dimension of one or more of the structure""s multiple layers may be altered to reinforce the identified portion. In particular, the material property/dimension of one or more of the layers may be altered in comparison to other (i.e., non-reinforced) portions of the multi-layered structure to produce the desired reinforcement. In a preferred embodiment of the subject aspect of the present invention, one or more layers of the structure are increased in thickness while one or more different layers of the structure are decreased in thickness to provide the desired reinforcement of the identified portion. For example, in the case of a composite structure having first and second face sheets with a core material sandwiched in-between, increasing the thickness of one or both face sheets while correspondingly reducing the thickness of the core may produce a portion of the structure having enhanced stiffness. Further, if the increase of the face sheet""s thickness is increased on its inside surface, the structurally enhanced portion will be free of any exterior irregularity associated with the reinforcement. Alternatively, a change in material properties, such as an increased density core or stiffer face sheets at the reinforced portion may be utilized to provide the desired reinforcement while the structure""s external surface remains free of any exterior irregularity. Once the materials/dimensions of each layer and the reinforced portion are determined, the structure may be formed by any of a number of methods known to those skilled in the art.
According to a fourth aspect of the present invention a composite structure having variable structural properties is provided. The composite structure comprises a first outermost layer, a second innermost layer and a core layer between the first and second layers creating what is often referred to as a composite sandwich structure. The outermost and innermost surfaces of the outermost and innermost layers, respectively, define the overall thickness of the composite structure at any position on the structure. The composite structure has a variation between a first set of structural properties at a first portion of the structure and a second set of structural properties at a second portion of the structure. This variation in structural properties is at least partially dependent on the relative proportions of the first layer, second layer and core layer at the first and second portions while remaining independent of the composite structure""s overall thickness, which may be the same at the first and second portions.
Various refinements exist of the features noted in relation to the subject third aspect of the present invention. These refinements and additional features may exist individually or in any combination. For example, the first and second layers may be made of any materials such as wood or metal. However, in a preferred embodiment the first and second layers comprise a plurality of fiber reinforced material layers such as carbon reinforced plastics, glass fibers, etc.
As noted, the variation in the structural properties of the composite structure between the first and second portions is at least partially related to the relative proportions of the thicknesses of the first layer, second layer, and core in these portions. For example, in a preferred embodiment, the thickness of one of the first and second layers may be increased in relation to the thickness of the core, which may itself be decreased in thickness such that the overall thickness of the structure remains unchanged. As will be appreciated, depending on the material properties (e.g., stiffness, density, etc) of the layers, this may result in a portion of the composite structure having enhanced structural properties in comparison to another portion of the composite structure that has different relative proportions and/or material properties of the layers. In this regard, a composite structure may have an equal overall thickness at a first and second portion while one of these portions provides enhanced or reduced structural properties in comparison to the other portion. For example, the compressive strength or stiffness of a particular portion may be increased or decreased to provide desired structural properties.
The materials utilized to form the layers may also be altered between the first and second portions to produce the variation between the first and second sets of structural properties. For example, where the first and second layer comprise a plurality of fiber reinforced material layers, a first portion may utilize a plurality of glass fibers layer while the second portion may utilize a mixture of glass and stiffer carbon fiber layers. In this regard the second portion may have an increased stiffness in comparison with the first portion. Alternatively, differing core materials may be utilized between the first and second positions having, for example differing densities to produce the variation between the first and second sets of structural properties.